1. Field of the Invention
This invention relates to a distributed control system wherein controlled objects provided in a plant are divided into a plurality of groups and each group is controlled by an independent controller.
2. Description of the Prior Art
In recent years, a distributed control system has come into wide use for plant control, the system being such that controlled objects provided in a plant are divided into a plurality of groups and an independent controller is provided for each group so that failure in the controller for one group will not affect the controller for another group. Also, availability of microprocessors at low cost has put into practice a more advanced distributed control system in which each of the controlled objects in one group is controlled separately from other controlled objects in the group by an individual closed-loop controller.
Typically, in such a distributed control system, the controllers are connected to a central processing unit through separate transmission paths and are centrally controlled by the central processing unit. In this system, in the event of failure of one controller, an object or a group of objects which have been controlled by the faulty controller are in a condition where they are without control, and in the extreme case, the plant as a whole may be affected adversely. To prevent such inconvenience, there has been proposed a system as disclosed in Japanese patent application No. 21273/77 entitled "Backup Control System" and filed on Feb. 28, 1977 in the name of Hokushin Denki Seisakusho, which application was laid open to the public on Sept. 16, 1978, under KOKOKU No. 106534/78. According to this proposal, each controller is connected not only to one object to be normally controlled by that controller through a transmission path but also to another object to be normally controlled by another controller through another transmission path in order to back up the latter controller, and in the event of failure of the backed up or guest controller, the host controller plays the part of the guest controller in controlling the other object associated therewith. In such a system, however, an additional transmission path must be provided between each controller and the other controlled object to be backed up thereby and when it is desired that each controller backs up a plurality of other controlled objects, the provision of a plurality of transmission paths is required between each controller and the other controlled objects to be backed up. This leads to a complicated system, and also involves the problem that each controller can back up only the controlled objects connected thereto through the additional transmission paths. It should be appreciated that the term "controlled object" used in this specification and the appended claims represents either one controlled object or one group of controlled objects to be controlled by one controller.